This invention relates to a process for recovering a radionuclide by separating it from a solution containing the radionuclide and its precursor.
Radioisotopes are commonly used for research, treatment and diagnosis in the field of nuclear medicine. Such applications include, for example, liver, lung, bone and tumor scanning and radiotherapy. In particular, positron emission tomography (PET) is a procedure in the field of nuclear medicine which offers advantages for radioimmunodiagnosis of cancer. PET requires radionuclides of high specific activity and high purity. Radionuclides are also in widespread use in both research and testing laboratories. In a significant fraction of these applications the radio-purity, chemical purity and specific activity of the radionuclides are critical to the successful outcome of the specific applications. Therefore, methodologies of the type described in this invention are essential to keep pace with the medical, scientific and engineering technologies in which they are experiencing an expanding demand.
Among the radionuclides which have been proposed for such applications are copper-62, scandium-47, copper-64 and copper-67. Cu-62 has been generated by use of a cyclotron to produce Zn-62, which then decays to Cu-62. Enriched Ca-46 has been irradiated to form Ca-47, which then decays to Sc-47. Copper-64, which is of special interest for use in PET, has been produced from zinc by fast neutron reaction involving the bombardment of a zinc target (48% natural abundance of Zn-64) as disclosed in Fritze, "The Preparation of High Specific Activity Copper 64", Radiochimica Acta 1964, 3:166-67 and Mirzadeh et al., "Production of No-Carrier Added Cu-67", Appl. Radiot. Isot., Vol. 37, No. 1, pp. 29-36, (1986). High-energy, or fast neutrons, having energies of 1 MeV or greater, can induce nuclear reactions that result in the ejection of a proton from the nucleus when the neutron is captured. Since the masses of the neutron and proton are nearly the same, the nominal mass of the transmutated product nuclide remains constant. However, since the captured neutron carries no charge and the ejected proton carries a positive one charge, there is a net loss of one positive charge. Hence the target atom which captured the neutron and ejected the proton is transmutated to the adjacent element in the periodic table having the atomic number one less than the target element. The importance of this nuclear transmutation process, as it relates to radioisotope production of the type serviced by this invention, is that transmutation of Zn-64 atoms produces radioactive Cu-64 atoms exclusively. Hence practical methodologies, such as this invention, that provide the chemical technology to separate the radioactive Cu-64 from the non-radioactive Zn-64 provide the means by which high-specific-activity radioisotopes can be produced. Zn-64(n,p)Cu-64 is the shorthand characterization for a stable Zn-64 atom capturing a neutron and ejecting a proton to produce radioactive Cu-64. Other examples of nuclear transmutation caused by high-energy neutron capture with proton ejection that are relevant to this invention are Zn-67(n,p)Cu-67 and Ti-47(n,p)Sc-47.
Radionuclides produced by bombardment reactions and decay reactions must be separated from their precursor nuclides before they can be used for radiodiagnosis and radiotherapy. Such separation is difficult because a radionuclide product is typically in a mixture containing 10,000 or more times as much of the precursor as of the radionuclide product. Methods which have been employed for radionuclide separation of this type include solvent extraction, electrodeposition and ion exchange.
Solvent extraction methods have been complicated and typically produce volumes of liquid organic waste contaminated with the precursor. Due to the employment of strong chelating agents, solvent extraction methods can result in product solutions contaminated with extraneous metal ions from reagents and other sources. Separation by electrodeposition involves the use of an electrolytic cell for repeated deposition and dissolution of the radionuclide product, for example, copper, onto an electrode. It has been reported Mirzadeh et al., "Production of No-Carrier Added Cu-67"Appl. Radiat. Isot., Vol. 37, No. 1, pp. 29-36, (1986), however, that the dissolved copper containing product must be subjected to an additional separation technique because electrodeposition is not effective for separation from impurities which are more electropositive than copper. Prior anion exchange chromatographic methods have not been consistently successful in minimizing breakthrough, that is, contamination of the product with precursor nuclide. Another disadvantage of such methods is that they typically require ion exchange columns with relatively large bed volumes.